The present invention relates to a method of forming a pattern by a lithographic process in accordance with a method of manufacturing a semiconductor device.
With recent decreases in design rules for a semiconductor device such as an IC or LSI, development has been directed toward a lithographic process using a light source of a shorter wavelength such as a KrF excimer laser (of a wavelength of 248 nm) or an ArF excimer laser (of a wavelength of 193 nm), which is for forming a minuscule pattern in a manufacturing process of a semiconductor device. In the lithographic process using a light source of a shorter wavelength, a chemically amplified resist embodying the concept of chemical amplification has generally been employed.
The chemically amplified resist is composed of a multicomponent-system substance containing an acid generator which generates an acid in response to the irradiation of an energy beam and a compound which reacts with the acid. The chemically amplified resist uses a reaction induced by an acid catalyst to change its dissolution property with respect to a developing agent, thereby enabling the formation of a minuscule resist pattern.
A description will be given to a conventional pattern formation method with reference to FIGS. 6(a) to 6(d).
First, as shown in FIG. 6(a), a BPSG film 2 as an insulating film is deposited to a thickness of 700 nm on a semiconductor substrate 1 and subjected to a thermal treatment at a temperature of 850.degree. C. under an atmosphere of flowing N.sub.2 gas as inert gas, so that the BPSG film 2 is caused to reflow. During the reflow process, a nitride layer 2a is formed from the N.sub.2 gas on the surface of the BPSG film 2.
Next, as shown in FIG. 6(b), a binary-system positive chemically amplified resist (such as WKR-PT-3 commercially available from Wako Pure Chemical Industries, Ltd.) is applied onto the BPSG film 2 by spin coating and subjected to pre-baking, thereby forming a resist film 5.
Next, as shown in FIG. 6(c), exposure to a KrF excimer laser 7 is performed using a mask 6, followed by post-exposure baking.
Next, as shown in FIG. 6(d), the resist film 5 is developed in an aqueous alkaline solution to provide a resist pattern 8.
In this case, the decomposition reaction of the compound caused by the acid generated from the chemically amplified resist proceeds in the exposed portion of the resist film 5. In other words, the reaction induced by the acid catalyst changes the alkali-soluble property of the chemically amplified resist, which enables the formation of a minuscule resist pattern.
However, in the case of forming a resist pattern on the BPSG film 2 by using the positive chemically amplified resist, footing is observed in the resist pattern 8 as shown in FIG. 6(d), so that a resist pattern having an excellent profile is not formed. Specifically, if the resist pattern 8 composed of the positive chemically amplified resist is formed on an insulating film having a reflowing property, such as the BPSG film 2, which has been deposited and caused to reflow for planarization at a temperature of about 800.degree. to 900.degree. C. under an atmosphere of flowing N.sub.2 gas, footing is observed in the resist pattern 8 so that a resist pattern having an excellent profile is not formed. If the resist pattern 8 composed of a negative chemically amplified resist is formed, on the other hand, undercut is observed in the resist pattern 8 so that a resist pattern having an excellent profile is not formed, either. The degraded profile and resolution of the resist pattern composed of the chemically amplified resist may adversely affect the subsequent process.
In the case of forming a resist pattern composed of a chemically amplified resist on a nitride film containing nitrogen atoms, such as a TiN film or SiN film, formed on a semiconductor substrate, the profile and resolution of the resist pattern are also degraded.
To prevent the foregoing degradation, there have been proposed a method of forming a Si thin film on a semiconductor substrate (see U.S. Pat. No. 5,219,788) and a method of forming an oxide film on a semiconductor substrate (see Japanese Laid-Open Patent Publication HEI 6-84774).
FIG. 7 shows the structure of a semiconductor device manufactured in accordance with the former one of the conventional methods. As shown in the drawing, a thin film 9 made of Si or SiO.sub.2 and containing Si is formed on a BPSG film 2 formed on a Si substrate and a resist pattern 8' is formed on the thin film 9.
However, it is difficult to implement the method of forming the thin film 9 containing Si on the surface of the BPSG film 2 because of a reduction in throughput caused by an increased number of process steps, a reduction in production yield caused by dust accompanying the increased number of process steps, and the necessity to etch a bi-layer film consisting of the thin film 9 containing Si and the BPSG film 2, which lead to lack of control and increased manufacturing cost.